Title says it all, but I also have some background info / requirements:
Related question: What happens if I add data from one coordinate system like WGS_1984_Web_Mercator_Auxiliary_Sphere on top of my basemap that uses a custom tiling scheme that uses NAD_1983_HARN_StatePlane_California_V_FIPS_0405_Feet? Would the proper projecting on-the-fly occur in the ArcGIS.com map viewer?
FYI - I am just looking for some best practices. So far I have the following info:
"When you use your own basemap, your map uses the projection of that basemap instead of Web Mercator, the projection of the map viewer."
from: "Use your own basemap" - https://doc.arcgis.com/en/arcgis-online/create-maps/choose-basemap.htm#ESRI_SECTION1_5EA1869127A74F1...
"The map viewer can't display layers in a map without a working basemap because the basemap establishes the coordinate system of the map. "
from: "Basemap considerations" - https://doc.arcgis.com/en/arcgis-online/create-maps/choose-basemap.htm#ESRI_SECTION1_E92C349BBC79476...
Using your own basemaps (and projection) with the ArcGIS.com map viewer - http://blogs.esri.com/esri/arcgis/2010/12/16/using-your-own-basemaps/
Web Mercator The Standard for Sharing Data on the Web - http://www.esri.com/events/seminars/bettermaps/~/media/files/pdfs/events/seminars/bettermaps/materia...